nmalinoski – isn’t game PCB sync output typically TTL? So, it really would be a simple direct connection for RGBS to a HD-15 the appropriate pins on AV3 (csync on pin13)?
I don’t really have experience with arcade hardware, but if it’s TTL RGBS like an SNES, then yeah; you could slice off the end of any old VGA cable, attach R, G, B, and S to the screw terminals (Good luck making a connection with ground/shield, though), and AV3 should take it.
The possible concern is if it’s not very-high-quality RGB output, like a lot of old home consoles. If that’s the case, it would be better to use a cable with components, or something like a supergun, to take advantage of the better low-pass filtering on AV1.